1. Field of the Invention
The present invention relates to an optical pickup for use in an apparatus adapted to record and/or reproduce information by radiating a convergent light beam to a recording layer of an optical information recording medium such as an optical disc which records the information on tracks, and more specifically to an optical system of such an optical pickup.
2. Description of the Related Art
In recent times, optical discs have been in wide use as means for recording/reproducing various data including video data, audio data and computer data. High density recordation type optical discs referred to as DVDs (Digital Versatile Disc), for example, are commercially available.
In a next generation optical disc system designed to increase a recording capacity, use of an optical disc recorded at an even higher density than the DVD together with an object lens having a numerical aperture NA as high as 0.85 in an optical pickup is suggested.
An optical disc has a light-transmissive cover layer as the outermost layer. The optical disc has one or more recording layers below the cover layer. If the numerical aperture of an object lens is increased, and the thickness of the cover layer to transmit light during an information recording/reproducing operation is deviated from a reference value, large spherical aberration results, and the spatial frequency characteristic (MTF) of the lens as expected cannot be achieved. When information is recorded/reproduced into/from short marks or pits in particular, unevenness in the cover layer thickness could increase an amount of jitter.
In order to solve the above described disadvantage, a beam expander which includes two lenses and servers as an optical device for correcting spherical aberration may be located between a light source and the object lens, so that the spherical aberration could be corrected. The beam expander converts a parallel light beam to a convergent or divergent light beam, and the resulting convergent or divergent beam is incident to the object lens. As a result, spherical aberration is included in a light emitted from the object lens. This spherical aberration compensates for another spherical aberration caused at the cover layer. If the cover layer thickness equals a reference value, the beam expander is operated to let out an incident parallel light beam as parallel, and slightly changes a light beam diameter. If the object lens is designed such that the spherical aberration caused at the object lens and the spherical aberration caused at the cover layer having the reference thickness just cancel each other, no spherical aberration is caused at a spot formed by a convergent light passing through the cover layer. If the thickness of the-cover layer is deviated from the reference value, the beam expander converts the parallel beam to a divergent or convergent beam. Specifically, if the cover layer is thinner than the reference thickness, the expander causes a convergent beam to be incident to the object lens. This increases an amount of spherical aberration caused at the object lens. The increased spherical aberration cancels a reduced amount of spherical aberration caused at the cover layer. A less amount of spherical aberration occurs at the cover layer because of the smaller cover layer thickness. Thus, there is no aberration on the information recording layer. Appropriate compensation is accordingly made.
FIG. 1 of the accompanying drawings illustrates a conventional optical pickup. When the spherical aberration is corrected with an optical system having an aperture located at an entrance pupil of the object lens as shown in FIG. 1, and a convergent (or divergent) beam is incident to the object lens, an emergent beam focuses at a position ahead of (or behind) the focal position of the object lens, so that the numerical aperture on the recording layer increases (or decreases). In other words, the numerical aperture fluctuates (NA1 greater than NA0 greater than NA2) on the recording layer as a result of the spherical aberration compensation. As the numerical aperture NA increases, the MTF expands to a level beyond the signal bandwidth, which increases noises in the signal. As the numerical aperture NA decreases, the MTF bandwidth necessary for reproducing the signal is not obtained, and a drop in the signal modulation factor degrades the SN ratio of the reproduced signal.
It is an object of the present invention to provide an optical pickup including an optical system that restricts or prevents changes in a numerical aperture on a recording layer even if spherical aberration is compensated for.
According to one aspect of the present invention, there is provided an optical pickup for use in an information recording and/or reproducing apparatus, comprising a condensing optical system having an object lens for condensing a light beam radiated from a light source on a recording layer of an information recording medium and an aperture positioned between the light source and the object lens. The apparatus records/reproduces information by radiation of the light beam condensed on the recording layer of the information recording medium. The aperture is positioned about the focal length of the object lens apart from the position of the principal incidence plane of the object lens toward the light source along an optical axis of the condensing optical system.
The condensing optical system may include an optical system capable of varying a light beam condensing position to correct spherical aberration by converting the light beam into a convergent or divergent beam and directing the beam to the aperture.